Apparatus and method for generating light beams of different output wavelengths using an opo, and opo

ABSTRACT

The apparatus for generating at least three visible light beams of different output wavelengths for display purposes comprises a passively mode-locked solid-state thin-disk laser and means for at least partially converting said primary light beam into electromagnetic radiation characterized by said at least three different output wavelengths including an optical parametric oscillator (OPO). The OPO is preferably an optical fiber feedback OPO. An optical fiber feedback OPO comprises a nonlinear optical element and feedback means for feeding back at least a proportion of the radiation emitted by the nonlinear medium to the nonlinear element, wherein the feedback means comprise an optical fiber.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to apparatus for producing electromagneticradiation of three different wavelengths, especially red, green and bluelight, using an optical parametric oscillator. It also relates to amethod for producing light of at least three different wavelengths usingan optical parametric oscillator.

[0003] Light sources of red, green and blue light (RGB systems) maycomprise a laser primary light source and an optical parametricoscillator (OPO) for nonlinear conversion. More specifically, the OPO isusually synchronously pumped with picosecond or femtosecond pulses. Thismeans that the OPO cavity usually must have the same length as thecavity of the primary laser or an integer fraction or multiple of thislength. The OPO cavity length must be precisely stabilized to match thelaser cavity length within typically a few tens of micrometers,depending on the pulse durations. Particularly with femtosecond pumppulses, the stabilization becomes rather critical and is seen as animportant drawback of synchronously pumped OPOs.

[0004] An OPO consists of a similar cavity as a laser resonator cavity,but the gain in the OPO is generated in a nonlinear crystal (e.g., madeof LBO, KTA etc.) which is pumped with the pulses from a laser. Thenonlinear crystal of the OPO generates a signal wave, for which the OPOcavity is resonant, and a corresponding idler wave. Alternatively, theOPO cavity may be resonant for the idler wave. The circulating signalpulses are synchronized with the pump pulses. The wavelengths of signaland idler waves are determined by phase matching, which depends on therefractive indices of the nonlinear crystal, i.e., on the material, itstemperature and the propagation directions. It has been shown (L. Lefortet al., Opt. Lett. 24 (1), 28, 1999) that the pulses generated by asynchronously pumped OPO can be more than 10 times shorter than the pumppulses.

[0005] 2. Description of the Related Art

[0006] High average power picosecond or femtosecond laser systems areknown in the art. WO 01/43242 discloses a so-called “thin-disk” laserbeing an ultrafast high average power system. A high average powerpicosecond laser system is described in Applied Phys. B, Vol. 17, pp.19-25 (2000).

[0007] U.S. Pat. No. 5,828,424 discloses a combination of conversionstages for generating three different visible light wavelengths using amode coupled Nd:YLF laser primary source with a wavelength of 1053 nm or1047 nm. The apparatus of this patent, however, suffers from severedrawbacks. For example, the schemes anticipated in the patent all usenonlinear crystals (e.g., KTA) and/or phase matching schemes thatrequire operation at elevated temperatures. Thus, elevated temperaturecontrol means are required. Also, the disclosed nonlinear crystals aresomewhat expensive and difficult to obtain and tend to be damaged if theapparatus is run for a long time with high power.

SUMMARY OF THE INVENTION

[0008] The goal of this invention is to provide a practical source ofred, green and blue light for display application which meets thefollowing requirements:

[0009] high output power, ideally >10 W in each color

[0010] high conversion efficiency, modest power and cooling requirements

[0011] compact setup with as few components as possible

[0012] stability of operation against vibrations, thermal drifts,changes in cavity length, etc.

[0013] durability (e.g., use materials with a minimum of long-termdegradation problems)

[0014] According to a first aspect of the invention, the apparatus forgenerating at least three light beams of different output wavelengthscomprises a passively mode-locked, solid-state thin-disk laser foremitting a primary light beam characterized by a primary beam effectivewavelength, means for at least partially converting said primary lightbeam into electromagnetic radiation having at least three differentoutput wavelengths, said means including an optical parametricoscillator (OPO).

[0015] According to a second aspect of the invention, the apparatus forgenerating at least three light beams of different output wavelengthscomprises a short-pulse, high average power laser for emitting a primarylight beam characterized by a primary beam effective wavelength, meansfor at least partially converting said primary light beam intoelectromagnetic radiation having at least three different outputwavelengths, said means including a fiber feedback optical parametricoscillator.

[0016] It should be noted that in the context of this application, theterm “light” always refers to electromagnetic radiation, not just in thevisible range but in other wavelengths as well, i.e., the term “light”refers to visible, infrared and ultraviolet electromagnetic radiation.

[0017] A method of generating at least three light beams of differentoutput wavelengths comprises the steps of producing a continuous trainof picosecond or sub-picosecond pulses of primary laser lightcharacterized by a primary beam effective wavelength, and at leastpartially converting, using non-linear optical elements, said primarylight beam into electromagnetic radiation characterized by said at leastthree different output wavelengths, wherein said at least partiallyconverting said primary light beam includes producing a signal beam andan idler beam in an Optical Parametric Oscillator (OPO), including thesteps of exciting, by laser light pumping, a non-linear optical elementplaced in an optical resonator to emit at least a signal beam and anidler beam of coherent electromagnetic radiation, and partially feedingback said idler beam into said non-linear optical element using anoptical fiber.

[0018] An OPO according to the invention (i.e. a device for emittingelectromagnetic radiation when being optically pumped by coherentelectromagnetic radiation having an input beam wavelength, wherein theinput beam wavelength and at least two output beam wavelengths of theemitted electromagnetic radiation are mutually different) comprises anonlinear optical element and feedback means for feeding back at least aproportion of the radiation emitted by the nonlinear medium to thenonlinear element, wherein the feedback means comprise an optical fiber.

[0019] A device for generating output electromagnetic radiationcharacterized by at least two different output beam wavelengthsaccording to the invention comprises a pumping beam pulse laser foremitting coherent electromagnetic radiation having a pumping beamwavelength, wherein the pumping beam wavelength and at least two outputbeam wavelengths are mutually different, a nonlinear optical elementplaced in the beam path of the pumping beam and feedback means forfeeding back at least a proportion of the radiation emitted by thenonlinear medium to the nonlinear element, wherein the feedback meanscomprise an optical fiber.

[0020] Finally, a method of generating electromagnetic radiationcharacterized by at least two different output beam wavelengths using acontinuous train of picosecond or sub-picosecond pulses of inputcoherent electromagnetic radiation having an input beam effectivewavelength, comprises the steps of: directing said input coherentelectromagnetic radiation onto an non-linear optical element, such thatoutput electromagnetic radiation is produced, and feeding back aproportion of said output electromagnetic radiation to said non-linearoptical element using a glass fiber, wherein said proportion of saidoutput electromagnetic radiation is characterized by a single one ofsaid output beam wavelengths.

[0021] The first key component of the apparatus according to theinvention is a high-average power picosecond or femtosecond lasersystem. This gives the advantage of having very high peak power for eachpulse with high-average power. High average power is taken to mean muchlarger than 10W, since, preferably, the output power of color shouldexceed 10W after several frequency conversion stages.

[0022] The preferred choice for a high-average power ultrafast system isa so-called “thin-disk” laser as described in WO 01/43242 or in therecent Optics Letters publication (Opt. Lett. Vol. 25, no. 11, 2000, pp.859-861), both publications being herein incorporated by reference.However, the invention also works with a high average power picosecondlaser system as described in Applied Phys. B. vol. 17, pp. 19-25, 2000or with any other high average power picosecond or femtosecond pulselaser.

[0023] In the above mentioned WO 01/43242, it has been shown that apassively mode-locked thin-disk laser can be designed for high averageoutput power. Such a passively mode-locked thin-disk laser features asolid-state laser gain medium with a cooling surface, the gain mediumbeing preferably in the shape of a thin plate and being mounted oncooling means. The means for passive mode-locking may include asemiconductor saturable absorber mirror (SESAM) device. The opticalresonator of the thin-disk laser may be designed such that the beam hitsthe thin-disk gain medium more than two times during each round-trip,whereby at least two hits with different angles of incidence occur suchthat a standing wave pattern in the thin-disk gain medium is at leastpartially eliminated.

[0024] A further key component of the invention is a fiber feedback OPO.This OPO solves the problem of critical cavity stability. This isachieved by the following measures: A very high parametric gain isprovided by the use of pump pulses with high peak power and/or the useof a highly nonlinear crystal. Then an OPO cavity is used where most ofthe generated signal light—and possibly idler light as well—is coupledout directly after the nonlinear crystal. The remaining light is coupledinto a (preferably single-mode) fiber, which can represent a largefraction of the total cavity length. The light transmitted through thefiber is then fed back into the nonlinear crystal.

[0025] A large fraction of the cavity length means that the product ofthe optical fiber refractive index and its lengths greatly exceeds thepath length of the beam in the cavity outside the fiber.

[0026] This device in the context of this application is always referredto as OPO, since it comprises a feedback, although compared to OpticalParametric Oscillators with conventional cavities, relatively littleradiation is fed back. According to the definition used throughout thisapplication, a device is an OPO if it comprises a nonlinear opticalmedium and some feedback.

[0027] This system has the following advantages (see also Opt. Lett. 26,304 (2001) and J. Phys. D. Appl. Phys. 34, 2433 (2001)):

[0028] Feedback with the fiber leads to a much more compact cavitysetup. The cavity length would usually be at least a few meters, whichleads to a large setup when the cavity is formed in the conventional waywith mirrors and free space propagation.

[0029] Because of the strong output coupler transmission, optical lossesin the feedback loop have only little influence on the power conversionefficiency of the OPO. This means that the system, compared withconventional OPO systems, is much less sensitive to losses which mightresult from fabrication errors or from aging of components. A highconversion efficiency is easily achieved.

[0030] The cavity length adjustment is also much less critical, even ifvery short pump pulses are used. This is because the very highparametric gain allows for efficient energy extraction, even if a cavitylength mismatch provides weak overlap of the feedback signal pulses withthe pump pulses in the crystal. For example, if the cavity length of theOPO is not perfectly matched to the primary laser, the pump pulse andthe seed pulse will not perfectly overlap. However the high gain of theparametric amplifier allows for efficiency energy extraction even underconditions of poor overlap.

[0031] The cavity length tolerance can be further enhanced by broadeningof the feedback pulses in the fiber due to nonlinear effects and/ordispersion. The OPO can still generate rather short signal pulses.

[0032] If the fiber parameters are suitably chosen, soliton pulsetransmission in the fiber can be obtained. This allows the generation ofshorter pulses, which may be desirable in cases where this facilitatesfurther frequency conversion. For soliton transmission, a fiber withlarge enough soliton energy is required. At high powers, this means thata fiber with relatively large mode area should be used.

[0033] As already mentioned, the fiber feedback OPO is ideally combinedwith a powerful picosecond or femtosecond primary laser. In particular,a passively mode-locked femtosecond thin disk laser is an ideal primarybeam source, because it provides a high average power without usingamplification stages, a good conversion efficiency, and rather shortpump pulses (e.g. 0.6 μs) which make efficient nonlinear conversioneasier. The relatively low repetition rate (e.g. 35 MHz), which wouldnormally lead to a quite large OPO setup, is no problem when a fiberfeedback OPO is used where the low repetition rate is easily obtained bysimply using a longer fiber.

[0034] The present invention encompasses a number of differentconversion schemes. A nonlinear conversion scheme for an RGB system isdefined by a certain combination of conversion steps and the wavelengthsinvolved, and the details on how the required conversion stages arerealized.

[0035] Desired features include: a small number of conversion stages, agood conversion efficiency, a compact and stable setup, flexibility togenerate the desired visible wavelengths, the use of nonlinear crystalswhich do not tend to degrade with time operation of all components at ornear room temperature, further power scalability.

[0036] In the following description, two schemes that can substantiallymeet the foregoing requirements are presented as preferred embodimentsof the invention. Of course, other RGB schemes can also be used.

[0037] According to a first embodiment, an incoming primary IR laserlight beam having a frequency of between 980 nm and 1100 nm is frequencydoubled in a second harmonic generating means (SHG) and then directedonto an OPO. The green light output is composed of light that isdirectly transmitted through the OPO, or alternatively, is composed of aproportion of the SHG output. The signal output beam and the idler are,together with residual infrared frequency light from the SHG, directedonto two subsequent Sum Frequency Generation (SFG) stages, where theblue and the red light, respectively, are created. Aspects of thisscheme have been described in the U.S. Pat. No. 5,828,424. However,although the possibility of a scheme of this type has been disclosed inthe mentioned US patent, it has not been disclosed that it can beimplemented exclusively with LBO crystals, which are all operated at ornear room temperature, with critical phase matching (except the last SFGstage where even noncritical phase matching near room temperature ispossible). The schemes anticipated in U.S. Pat. No. 5,828,424 allcomprise the use of other nonlinear crystals (e.g., KTA) and/or phasematching schemes which require the operation at elevated temperatures.

[0038] The use of LBO with critical phase matching schemes has thefollowing advantages:

[0039] Temperature-controlled ovens are not required, because allcrystals can be operated at or near room temperature.

[0040] LBO crystals have been proven to be very damage resistant andthus promise to be suitable for long-term operation with high outputpowers without significant degradation of the frequency conversion.

[0041] LBO crystals are commercially available from various sources, andthe prices are moderate even if large crystal sizes are needed. Themoderate cost of large LBO crystals is important because it leaves a lotof room for further power scaling where larger beam diameters will berequired.

[0042] However, such a scheme can be efficient only if a pump sourcewith a rather large peak power is used. This is due to the relativelysmall nonlinearity of LBO crystals and the use of critical phasematching schemes that do not permit the use of strongly focused beams.While such restrictions may have prevented other people in the fieldfrom devising such a scheme, it can be fulfilled with the recentlydemonstrated passively mode-locked thin disk laser. This laser providesnot only high average output powers, but also pulses which are about anorder of magnitude shorter than pulses from other high-power mode-lockedsources. This leads to very high peak powers in the megawatt domainwhere the above described nonlinear conversion scheme can be operatedefficiently.

[0043] According to a second embodiment, the conversion scheme is set upas follows:

[0044] An OPO, directly pumped with a mode-locked laser at infraredwavelengths, generates a signal and idler output with wavelengths around1.8 μm and 2.6 μm, respectively.

[0045] Blue light is generated by frequency doubling the signal outputaround 1.8 μm in two subsequent frequency doubling means.

[0046] Red light is generated by frequency doubling the idler outputaround 2.6 μm in two subsequent frequency doubling means.

[0047] Green light is obtained from an additional frequency doublingmeans, directly pumped with the primary laser. The residual infraredlight from the frequency doubling means may actually be used to pump theOPO, instead of splitting the infrared pump beam.

[0048] This scheme, which has not been previously described, has theadvantage that no sum frequency mixers are used that require pulses fromtwo input beams to overlap spatially and temporally. Also, the schemeoperates most of the conversion stages at long wavelengths where damageproblems are less severe or even not likely to occur.

[0049] A preferred embodiment of an apparatus according to the inventioncomprises:

[0050] a passively mode-locked laser with high average power and short(preferably sub-picosecond) pulse duration, preferably a passivelymode-locked thin disk laser

[0051] means for at least partially converting said primary light beaminto electromagnetic radiation having at least three different outputwavelengths, wherein the only nonlinear crystals comprised in said meansbeing LBO crystals which can be operated at or near room temperature.preferably a fiber feedback OPO, forming the central part of thementioned means for converting said primary light beam.

[0052] Such a system has the following advantages:

[0053] It contains a minimum number of components and is compact becauseit does not need any amplification stages and no temperature-controlledcrystal ovens. It is also probably less expensive to manufacture,compared to other known systems.

[0054] It is compact and stable. The system is stable against smallchanges in the synch-pumped OPO cavity length, and losses within the OPOcavity, for example.

[0055] It can be scaled to very high output powers.

[0056] It has a very good power conversion efficiency because the pumplaser is efficient and the conversion scheme allows the unconvertedlight from a part of the conversion stages to be recycled. This leads tomoderate demands on electric power and cooling water.

[0057] It relies on easily available crystals which are very highlyresistant to long-term degradation with time when operated with higherpowers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058]FIG. 1 is a schematic top plan view of thin disk pump laser of anapparatus according to the invention.

[0059]FIG. 2 is a scheme of a first embodiment of an apparatus accordingto the invention.

[0060]FIG. 3 is a scheme of a second embodiment of an apparatusaccording to the invention.

[0061]FIG. 4 is a scheme of a variant of the second embodiment of anapparatus according to the invention.

[0062]FIG. 5 is a schematic top plan view of a fiber feedback OPO of anapparatus according to the invention.

[0063]FIGS. 6 through 8 represent results of measurements made forcharacterizing a prototype fiber feedback OPO of the kind represented inFIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] In the following description, preferred embodiments of theinvention are described with reference to the figures. It should benoted that the given dimensions, wavelengths, materials temperatures,etc. are mere examples and by no way limit the scope of the invention.

[0065]FIG. 1 shows a schematic, simplified illustration of a laser 1according to the invention. The laser 1 comprises an optical resonatordelimited by a first reflective element 11 and a second reflectiveelement 12 for reflecting laser radiation 10. In the preferredembodiment of FIG. 1, the first reflective element 11 is a semiconductorsaturable absorber mirror (SESAM) device 4 for passively mode lockingthe laser; it is described below with reference to FIG. 3. In otherembodiments, the first reflective element 11 could be, for example, adielectric mirror. The second reflective element 12 may, for example, bea partially reflecting outcoupling dielectric and/or metallic mirror.

[0066] The resonator shown in FIG. 1 is multiply folded by severalfolding mirrors 13.1-13.8. Their radii may be the following:R₁=R₃=R₅=R₆=R₇=∞; R₂=R₈=1.5 m; R₄=1 m. The total length of the geometricpath in the resonator 1 in this example is 10 m, corresponding to arepetition rate of 15 MHz. The lengths of the portions between thefolding mirrors 13.1-13.8 may be calculated by comparison with FIG. 1since FIG. 1 shows the lengths of the portions in a correct scale. Ofcourse, many other laser resonator designs are possible for the pumplaser.

[0067] A thin-disk laser head comprising a thin-disk solid-state lasergain medium 2 mounted on cooling means 3 is placed inside the opticalresonator. The thin-disk laser head simultaneously acts as a foldingmirror 13.6. Alternatively, the thin-disk laser head could be used as anend mirror 11 in the resonator cavity. The laser gain medium 2 ispreferably optically pumped by light emitted by a laser diode (notshown) and impinging on a front surface of the thin disk 2. For purposesof clarity, means for exciting the laser gain medium 2 are not shown inFIG. 1.

[0068] The thin disk laser gain medium is preferably made of Yb:YAG.This laser gain material offers a very good efficiency of typically 50%and allows the generation of very short pulses with durations even below1 μs. Other possible laser gain materials are, e.g., Yb:KGW, Yb:KYW,Nd:YAG, Nd:YVO₄ (neodymium vanadate), or semiconductors.

[0069] The laser of FIG. 1 still further comprises a Gires-Tournoisinterferometer (GTI) 5 as a dispersion-compensating means. The GTI 5simultaneously acts as a folding mirror 13.3. An optional glass plate 6is placed inside the optical resonator and oriented such that the angleof incidence of the laser beam 10 is equal to the Brewster angle α_(B)in order to obtain a linear polarization of the laser beam 10. The glassplate 6 could simultaneously be used for Kerr lens mode locking (KLM).Alternatively, the Kerr effect needed for KLM could be provided by anadditional plate, by the laser gain medium 2 or by the GTI 5.

[0070] The primary beam source laser has the following parameters:

[0071] 50-100 W average power at 1030 nm

[0072] beam quality: M²<1.5

[0073] pulse duration: 0.5-3 μs

[0074] repetition rate: 10-100 MHz

[0075] The design of the pump laser can, for example, be as described inOpt. Lett. 25, 859 (2000).

[0076] The apparatus of FIG. 2 comprises a pulsed laser 1 serving as aprimary laser. In the described embodiment, the laser 1 emits infraredradiation with a wavelength of 1030 nm. The laser 1 is preferably, butnot necessarily, a thin disk laser, e.g. of the kind described withreference to FIG. 1. The laser 1 may also be any other high averagepower picosecond or femtosecond pulse laser. A frequency doubling means21, i.e. nonlinear crystal serving as second harmonic generation means,is placed in the output beam direction of the laser 1. The crystal may,for example, be a lithium triborate (LBO) crystal operated at or nearroom temperature.

[0077] For some applications, the wavelengths mentioned here are tooshort. This problem can be overcome by choosing a primary beamwavelength of about 1045 nm to 1080 nm, which can be obtained with amodified laser crystal material.

[0078] The transmitted 1030 nm primary light beam proportion isseparated from the frequency doubled 515 nm light using a firstwavelength selective element 22, which may, for example, be a mirrorwith a frequency dependent transparency/reflectivity. The 515 nmfrequency doubled light beam is used as a pumping beam of an OPO 23.

[0079] An example of an OPO is described further below with reference toFIG. 5.

[0080] From the OPO, light beams with three different wavelengths areemitted. The transmitted proportion of the OPO 515 nm pumping beam isdirected to an output of the apparatus and serves as green light source.A 780 nm signal beam and a 1516 nm idler beam are led together with the1030 nm primary light beam proportion which was transmitted through thefrequency doubler 21. For this purpose, a second mirror 24 with afrequency dependent transmittivity/reflectivity may be used. Theresulting beam is guided to a first sum frequency generating means 25,where a 444 nm blue output beam is produced from 780 nm and 1030 nm beamproportions. The first sum frequency generating means 25 comprises anLBO crystal which can be operated at or near room temperature and withcritical phase matching.

[0081] A third wavelength selective element 26 separates the 444 nmoutput beam from the other beam proportions and directs it to an outputlocation. A second sum frequency generating means 27 produces a 613 nmred output beam from the 1030 nm and 1516 nm beam portions. The secondsum frequency generating means 27 may comprise an LBO crystal which canbe operated at or near room temperature with critical phase matching oreven with non-critical phase matching.

[0082] Even if phase matching at room temperature is possible, one mightoperate the nonlinear crystal in a device which stabilizes itstemperature somewhere near room temperature (such as within 40 K of roomtemperature), ideally at a somewhat elevated temperature like 40 degreesCelsius. This can be done, for example, with a Peltier element, and doesnot require a sophisticated oven construction, such as would be requiredto operate at a higher temperature like 200 degree Celsius. The slightlyelevated temperature helps to avoid any problems with moisture and alsoallows the device to operate properly even if the ambient temperature issomewhat increased, for example, by excess heat dissipated by othercomponents of the apparatus. Thus, phase matching near room temperatureis advantageous, even if one still decides to operate at a slightlyelevated temperature.

[0083] The method of generating at least three visible light beams ofdifferent output wavelengths for display purposes using this embodimentof the invention can be summarized as follows:

[0084] Step 1: Second Harmonic Generation in the frequency doublingmeans 21: 1030 nm→515 nm (green)

[0085] Step 2: OPO: 515 nm→780 nm+1516 nm (signal+idler)

[0086] Step 3: Sum Frequency Generation in the first sum frequencygeneration means 25: 1030 nm+780 nm→444 nm (blue)

[0087] Step 4: Sum Frequency Generation in the second sum frequencygeneration means 27 1030 nm+1516 nm→613 nm (red)

[0088] The order of the two SFG stages could be reversed. However, thegiven order seems to be good because the second stage has to operatewith a lower 1030 nm pump power, and the efficiency is less critical inthe stage for the red output because of the noncritical phase matchingscheme.

[0089] One OPO mirror could be mounted on a piezo (or another movablepart) in order to install a cavity length stabilization scheme.

[0090] As already mentioned, the involved wavelengths can be modified toadapt to other primary laser wavelengths or to the output wavelengthsrequired from the concrete system.

[0091] A scheme of a second apparatus according to the invention isshown in FIG. 3. The apparatus comprises a laser primary beam source 1,an OPO 23 and five nonlinear crystals 41, 42, 43, 44, 45 for serving asfrequency doubling means. These components, as well as other opticalelements, are similar to the components described with reference to FIG.2 and are not again described here. The mode of operation of theapparatus as well as the corresponding method may be summarized asfollows: The OPO 23, directly pumped with a mode-locked laser atinfrared wavelengths, generates a signal and idler output withwavelengths around 1.8 μm and 2.6 μm, respectively or with wavelengthvalues slightly below. Blue light is generated by frequency doubling thesignal output around 1.8 μm in two subsequent frequency doubling means42, 43. Red light is generated by frequency doubling the idler outputaround 2.6 μm in two subsequent frequency doubling means 44, 45. Greenlight is obtained from an additional frequency doubling means 41,directly pumped with the primary laser.

[0092] As an alternative to this set-up, the residual infrared lightfrom the frequency doubling means 41 for obtaining green light mayactually be used to pump the OPO, instead of splitting the infrared pumpbeam. This variant is depicted in FIG. 4.

[0093] Also, the frequency doubling means 42, 43 for generating bluelight and the frequency doubling means 44, 45 for generating red lightmay, in principle, be identical, i.e. only one pair of frequencydoubling means is present after the OPO, through which both the signaland the idler beam are directed. Finally, starting from the set-up ofFIG. 3, the frequency doubling means for generating green light may beomitted by using the first of the frequency doubling means following theOPO and directing the residual pumping beam of the OPO through it andgenerating green light out of it. In order to obtain phase matching fordifferent wavelengths simultaneously, periodically poled crystals withdifferent grating periods would have to be produced in a single crystal.

[0094] An example of an OPO according to the invention is shown in FIG.5. The pump beam is focused with a curved mirror 31 to a waist with 90μm radius in the middle of a nonlinear crystal 32. The crystal 32 mayalso be an LBO crystal which can be operated at or near roomtemperature, with critical phase matching or non-critical phasematching. Alternatively, the crystal may be a Periodically Poled LiTaO₃(PPLT) crystal, a KTA crystal or any other known nonlinear crystalusable in an OPO (see also the discussion below). The crystal length isbetween 2 mm and 40 mm. After the nonlinear crystal, the signal wave iscollimated by a second and a third mirror 33, 34. A further element 35which is partially reflective for light of the signal wavelengthseparates a proportion of the signal beam from the other proportion ofthe signal beam, the idler beam and possible pumping beam proportions.The second 33 or the third 34 mirror may, as an alternative to thesketched set-up, be transparent for light of the pumping beam wavelengthand possibly also of the idler beam wavelength, so that this mirrorserves for coupling out the pumping beam proportion which is transmittedto the crystal and/or the idler beam.

[0095] The beam proportion reflected by the element 35 is used for thesignal feedback, while the transmitted beam proportions represent theoutput. The feedback light is launched, via a first lens 36, into a1.5-10 m long standard telecom fiber 37, which is single-mode at thesignal wavelength. The light emerging from the fiber is mode-matched bytwo lenses 38, 39 and fed back into the crystal through a dichroicmirror 40, which is highly reflective for the pump wave and transmissiveat the signal wavelength.

[0096] A notable feature of the fiber-feedback OPO, which results fromthe high gain and strong output coupling, is the insensitivity of theperformance to cavity losses. In an experiment with a similar set-up,the maximum output power was shown to be reduced by only 6% if anadditional filter with 10 dB loss at the signal wavelength was insertedat the fiber launch between the element 35 and lens 36. Obviously, it isnot necessary to minimize the losses in the feedback loop after theoutput coupling.

[0097] Another favorable factor, relaxing the tolerances, is possiblebroadening of the feedback pulses in the fiber due to nonlinear effectsand/or dispersion. The OPO can still generate rather short signalpulses.

[0098] The fiber-feedback OPO according to the invention may be used ina system of the kind described above, i.e. in an apparatus forgenerating red, green and blue light. However, it may used in any othersetup where frequency conversion is an issue. As an example, it may beused to generate red light using green light (e.g., 515 nm inputradiation −>a 642 nm signal ouput beam (red) and a 2603 nm idler outputbeam). It may, as another example, also be used to generate 1.55 μmwavelength pulses for data transmitting purposes. The expert will knowmany other examples where conversion of a light beam into a light beamof a different, longer wavelength is desired. The fiber feedback OPOaccording to the invention may also be used to modify the pulse shape ofinput pumping or primary beam pulses.

[0099] Measurement results are shown in FIGS. 6 through 8 for an OPO ofthe kind described with reference to FIG. 5 and further comprising afilter element 39 in the beam path. The Yb:Y AG pump laser generatespulses with a duration of 0.6 μs at a repetition rate of 35 MHz, anddelivers a pump power of 8.2 W. The achieved signal power output isbetween 2.3 W and 2.7 W. The crystal is a poled LiTaO₃ (PPLT) crystal,which has a relatively high nonlinearity. The 22 mm long and 0.5 mmthick, uncoated crystal is operated at a temperature of 150° C. to avoidphotorefractive damage. The OPO signal wavelength depends on the periodof the poling pattern and the crystal temperature. The crystal has 8poled regions of transverse width 1.2 mm, with different grating periodsof 28.3 μm −29 μm, resulting in signal wavelengths between 1429 nm and1473 nm (for 150° C. crystal temperature).

[0100]FIG. 6 shows the typical performance for one grating. As shown,the maximum output power is reduced by only 6% when an additional filterwith 10 dB loss at the signal wavelength is inserted at the fiber launchbetween the element 35 and lens 36. This demonstrates the previouslymentioned insensitivity to cavity losses.

[0101] The duration of the signal pulses is measured by auto intensitycorrelation as shown in FIG. 7. For all poled channels and pump powers,the pulse duration (FWHM) is typically around 700-900 fs assuming anideal sech2 pulse shape. The spectral width is around 3-4 nm (FWHM),leading to a time-bandwidth product 0.360.53. For 2.5 W signal power at1429 nm (grating period 28.3 μm), we obtain 870-fs pulses with aspectral width of 2.8 nm, leading to a time-bandwidth product of 0.36,which is not far from the Fourier limit.

[0102] Despite the short pulse duration, the adjustment of thefiber-feedback OPO cavity length is not critical because of the highparametric gain. For example, even if the leading edge of a signal pulseis temporally overlapped with the pump pulse in the crystal, the highparametric gain still allows for efficient energy extraction. Also,nonlinear effects in the fiber can lead to a substantial temporalbroadening of the seed pulses. FIG. 8 shows that varying the round-triplength of the resonator over a range of 0.5 mm (corresponding to morethan one FWHM pulse width) led to an output power reduction by less than10%. Within this range, the pulse duration did not change significantly.The central wavelength of the optical spectrum changed less than 0.5 nmand the bandwidth less than 0.3 nm. The operation of the fiber-feedbackOPO system is stable over hours, and no signs of crystal damage wereobserved during all experiments.

[0103] Numerous other embodiments may be envisaged, without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. An apparatus for generating at least three lightbeams of different output wavelengths comprising a passively mode-lockedsolid-state thin-disk laser for emitting a primary light beamcharacterized by a primary beam effective wavelength, means for at leastpartially converting said primary light beam into electromagneticradiation characterized by said at least three different outputwavelengths, said means including an optical parametric oscillator(OPO).
 2. The apparatus according to claim 1, wherein said opticalparametric oscillator (OPO) is a fiber feedback optical parametricoscillator.
 3. The apparatus according to claim 1 or 2, wherein saidoptical parametric oscillator comprises an LBO nonlinear crystal.
 4. Theapparatus according to claim 1 or 2, wherein said primary beam effectivewavelength is between 980 nm and 1100 nm.
 5. The apparatus according toclaim 1 or 2, wherein said means for at least partially converting saidprimary light beam further comprise at least one frequency doublingmeans and at least one sum frequency generating means.
 6. The apparatusaccording to claim 5, wherein said primary light beam source, saidoptical parametric oscillator (OPO), at least two frequency doublingmeans, light deflecting means and dichroic elements are mutuallyarranged in a manner that, if the primary light beam source emitselectromagnetic radiation characterized by an effective wavelengthbetween 980 nm and 1100 nm, a first frequency doubling means producesgreen light by frequency doubling light of the primary light beam thethus frequency doubled light is used for pumping said OPO, the residualpumping light of said OPO serving as green light output, (one could alsotake a portion of the green light before the OPO) blue light isgenerated by a first frequency sum generation means (25), of which thesignal output of said OPO and residual primary light from said firstfrequency doubling means serve as input, and red light is generated by afrequency sum generation means (27), of which the idler output of saidOPO and residual primary light from said first frequency doubling meansserve as input.
 7. The apparatus according to claim 1 or 2, wherein saidprimary light beam source, said optical parametric oscillator (OPO), atleast two frequency doubling means, light deflecting means and dichroicelements are mutually arranged in a manner that, if the primary lightbeam source em its electromagnetic radiation characterized by aneffective wavelength between 980 nm and 1100 nm, The OPO is directlypumped with said primary light beam and generates a signal and idleroutput, Blue light is generated by frequency doubling the signal outputin two subsequent frequency doubling means, Red light is generated byfrequency doubling the idler output in two subsequent frequency doublingmeans, Green light is obtained from a frequency doubling means which isdirectly pumped with a primary beam proportion.
 8. The apparatus ofclaim 7, wherein said frequency doubling means for obtaining green lightare first frequency doubling means being pumped with the primary lightbeam and wherein the residual primary light beam proportion from saidfirst frequency doubling means is used for pumping the OPO.
 9. Theapparatus of claim 1 or 2 wherein at least some of the nonlinear opticalelements are of a kind which allows operation at or near roomtemperature.
 10. The apparatus of claim 9, wherein all of the nonlinearoptical elements contained in the apparatus are chosen so that operationat or near room temperature is possible.
 11. The apparatus of claim 9wherein at least some of the nonlinear optical elements are LBOcrystals.
 12. The apparatus of claim 11, wherein all nonlinear opticalelements contained in the apparatus are LBO crystals.
 13. An apparatusfor generating at least three visible light beams of different outputwavelengths comprising a short-pulse high average power laser foremitting a primary light beam characterized by a primary beam effectivewavelength, means for at least partially converting said primary lightbeam into electromagnetic radiation characterized by said at least threedifferent output wavelengths, said means including a fiber feedbackoptical parametric oscillator (OPO).
 14. The apparatus according toclaim 13, wherein the laser for emitting a primary light beam comprisesmeans for generating sub-picosecond pulses.
 15. The apparatus accordingto claim 13 or 14, wherein the laser for emitting a primary light beamis designed in a manner that it can be operated at average output powersexceeding 50 W.
 16. The apparatus according to claim 13 wherein at leastsome of the optically nonlinear elements are of a kind which allowsoperation at or near room temperature.
 17. The apparatus according toclaim 16, wherein all of the nonlinear optical elements of the apparatusare of a kind which allows operation at or near room temperature. 18.The apparatus according to claim 13 wherein at least some of thenonlinear optical elements are LBO crystals.
 19. The apparatus of claim16, wherein all nonlinear optical elements of the apparatus are LBOcrystals.
 20. The apparatus according to claim 13, wherein said primarylight beam source, said optical parametric oscillator (OPO), at leasttwo frequency doubling means and light deflecting means and dichroicelements are mutually arranged in a manner that, if the primary lightbeam source emits electromagnetic radiation characterized by aneffective wavelength between 980 nm and 1100 nm, a first frequencydoubling means produces green light by frequency doubling light of theprimary light beam the thus frequency doubled light is used for pumpingsaid OPO, the residual pumping light of said OPO serving as green lightoutput, blue light is generated by a first frequency sum generationmeans (25), of which the signal output of said OPO and residual primarylight from said first frequency doubling means serve as input, and redlight is generated by a frequency sum generation means (27), of whichthe idler output of said OPO and residual primary light from said firstfrequency doubling means serve as input.
 21. The apparatus according toclaim 13, wherein said primary light beam source, said opticalparametric oscillator (OPO), at least two frequency doubling means andlight deflecting means and dichroic elements are mutually arranged in amanner that, if the primary light beam source emits electromagneticradiation characterized by an effective wavelength between 980 nm and1100 nm, The OPO is directly pumped with said primary light beam andgenerates a signal and idler output, Blue light is generated byfrequency doubling the signal output in two subsequent frequencydoubling means, Red light is generated by frequency doubling the idleroutput in two subsequent frequency doubling means, Green light isobtained from a frequency doubling means which is directly pumped with aprimary beam proportion.
 22. The apparatus of claim 21, wherein saidfrequency doubling means for obtaining green light are first frequencydoubling means being pumped with the primary light beam and wherein theresidual primary light beam proportion from said first frequencydoubling means is used for pumping the OPO.
 23. The apparatus accordingto claim 13, wherein said primary beam effective wavelength is between980 nm and 1100 nm.
 24. A method of generating at least three lightbeams of different output wavelengths comprising the steps of Producinga continuous train of picosecond or sub-picosecond pulses of primarylaser light characterized by a primary beam effective wavelength, and atleast partially converting, using non-linear optical elements saidprimary light beam into electromagnetic radiation characterized by saidat least three different output wavelengths, wherein said at leastpartially converting said primary light beam includes producing a signalbeam and an idler beam in an Optical Parametric Oscillator (OPO)including the steps of exciting, by laser light pumping, a non-linearoptical element placed in an optical resonator to emit at least a signalbeam and an idler beam of coherent electromagnetic radiation, andpartially feeding back said idler beam into said non-linear opticalelement using an optical fiber.
 25. The method of claim 24, wherein saidprimary beam effective wavelength is between 980 nm and 1100 nm.
 26. Themethod according to claim 25, wherein green light is produced byfrequency doubling light of the primary light beam the thus frequencydoubled green light is used for pumping said OPO, the residual pumpinglight of said OPO serving as green light output, blue light is producedby, using a non-linear optical element and phase matching techniques,generating a sum frequency beam of the signal output of said OPO andresidual primary light from said first frequency doubling means, and redlight is produced by, using a non-linear optical element and phasematching techniques, generating a sum frequency beam of the idler outputof said OPO and residual primary light from said first frequencydoubling means.
 27. The method according to claim 25, wherein, The OPOis directly pumped with said primary light beam and generates a signaloutput and an idler output, Blue light is generated by twice frequencydoubling the signal output, Red light is generated by twice frequencydoubling the idler output, Green light is obtained by frequency doublinga primary beam proportion.
 28. The method according to claim 27, whereinthe residual primary light beam proportion from producing the greenlight is used for pumping the OPO.
 29. The method according to claim 24wherein at said non-linear optical elements are operated at or near roomtemperature.
 30. A device for emitting electromagnetic radiation whenbeing optically pumped by coherent electromagnetic radiationcharacterized by an input beam wavelength, the input beam wavelength andat least two output beam wavelengths of the emitted electromagneticradiation being mutually different, the device comprising a nonlinearoptical element and feedback means for feeding back at least aproportion of the radiation emitted by the nonlinear medium to thenonlinear element, wherein the feedback means comprise an optical fiber.31. The device according to claim 30, wherein the nonlinear medium hastwo end faces, radiation characterized by said at least two output beamwavelengths being emitted through a first one of said end faces, and thefeedback means and the nonlinear element being arranged in a manner thatradiation emitted through said first end face is fed back through thesecond one of said two end faces.
 32. The device according to claim 30or 31, wherein said optical fiber is a single mode optical fiber. 33.The device according to claim 32, wherein the diameter, the refractiveindex and in-coupling means of said optical fiber are such thatradiation characterized by the shorter one of said at least two outputwavelength—the signal wavelength—is fed back to the nonlinear medium.34. The device according to claim 30 or 31, wherein the fiber is astandard telecom optical glass fiber.
 35. The device according to claim30 or 31 not comprising any actively controlled means for stabilizingthe length of the feedback path.
 36. The device according to claim 30 or31 the nonlinear optical element being a χ² nonlinear optical element.37. A device for generating output electromagnetic radiationcharacterized by at least two different output beam wavelengthscomprising A pumping beam pulse laser for emitting coherentelectromagnetic radiation characterized by an pumping beam wavelength,the pumping beam wavelength and said at least two output beamwavelengths being mutually different, a nonlinear optical element placedin the beam path of the pumping beam and feedback means for feeding backat least a proportion of the radiation emitted by the nonlinear mediumto the nonlinear element, wherein the feedback means comprise an opticalfiber.
 38. A method of generating electromagnetic radiationcharacterized by at least two different output beam wavelengths using acontinuous train of picosecond or sub-picosecond pulses of inputcoherent electromagnetic radiation characterized by a input beameffective wavelength, comprising the steps of Directing said inputcoherent electromagnetic radiation onto an non-linear optical element,such that output electromagnetic radiation is produced, and Feeding backa proportion of said output electromagnetic radiation to said nonlinearoptical element using a glass fiber, wherein said proportion of saidoutput electromagnetic radiation is characterized by a single one ofsaid output beam wavelengths.
 39. The method according to claim 38,wherein the non-linear optical element has two end faces, radiationcharacterized by said at least two output beam wavelengths being emittedthrough a first one of said end faces, and wherein radiation emittedthrough said first end face is fed back through the second one of saidtwo end faces.